The present invention relates to a metal gasket such as a gasket for an exhaust manifold for an engine, or a cylinder head gasket.
When a joint surface between an exhaust manifold and an exhaust pipe for an automobile engine, or a joint surface between a cylinder head and a cylinder block (cylinder body) is sealed, a metal gasket is clamped between these members to seal combustion gas, coolant water, and lubrication oil.
Such a metal gasket is made by a design method, in which, mainly as sealing means, a full bead with a projected cross-section and a half bead with a step-like (crank-like) cross-section are arranged near the perimeter of a seal target hole. Among them, the half bead can be produced at a low cost, and easily sealed by a small fastening force, so that it is widely used for gasoline engines with a small fastening force compared to diesel engines.
With respect to the metal gasket with this half bead, there is a case in which a straight line portion is created in the gasket for the exhaust manifold when the gasket is seen in plan depending on the shape of an attached member. This straight line portion has proven to cause a problem such as water leakage or oil leakage during engine operation as compared to a curved line portion. It is commonly believed that the reason the straight line portion easily creates the above-mentioned problem is that a creep relaxation in the straight line portion of the half bead increases during the operation of an engine wherein the gasket is equipped, so that the seal surface pressure partially decreases.
Therefore, when the half bead forms a straight line in a plan view, if the straight line part extends, the half bead has less resistance to a compressive force compared to the full bead. Accordingly, the creep relaxation is generated in this half bead, so that the seal quality cannot be fully exerted.
For one solution for the above-mentioned problem, metal gaskets, such as those shown in Japanese Patent Publication No. 2004-92475, are formed by arranging a pair of half beads symmetrically in a thickness direction around a liquid hole which is formed substantially in a square or a rectangular shape. Also, outlines of corner parts of the half beads are formed in a shape with a radius which is larger than that of a corner part of the liquid hole. As a whole, the metal gaskets have a shape extending in a smooth circular shape. However, in order to form this kind of half beads, a space is required around the liquid hole (seal target hole), so that they are not practical.
On the other hand, even in the cylinder head gasket, as the weight and size of an engine have been reduced, an engine member tends to have a lower rigidity, and the deformation volume of the cylinder head which repeatedly occurs by engine operation has increased. Accordingly, a force compressing the half bead also increases, so that the serious problem is a creep relaxation of the half bead.
In addition, in the metal gaskets, the tightening pressure differs depending on the distance from the tightening bolt bore, so that the seal surface pressure generated in the half beads is also uneven. As a result, the creep relaxation occurs sporadically.
In view of the problems described above, the present invention has been made such that the metal gasket prevents the creep relaxation in the half bead formed around the seal target bore of the metal gasket. Also, the metal gasket widens the distribution of the surface pressure generated in the half bead and prevents a scratch in the member clamping the metal gasket, hereby securing an excellent seal quality.
Further objects and advantages of the invention will be apparent from the following description of the invention.